Raman phonons as a probe of disorder, fluctuations, and local structure in doped and undoped orthorhombic and rhombohedral manganites

Abstract

We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular, we study RMnO 3 (RϭLa, Pr, Nd, Tb, Ho, Er, Y, and Ca͒ and the different phases of Ca-or Sr-doped RMnO 3 compounds as well as cation deficient RMnO 3 . The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of MnO 6 octahedra and independently of the average structure obtained by diffraction techniques. One type of spectra corresponds to the orthorhombic Pbnm space group for octahedra with cooperative or dynamic Jahn-Teller distortions, with stretching modes as the main features and whose frequencies correlate to Mn-O distances. The other spectrum is associated to regular but tilted octahedra whose modes can be described in the rhombohedral R3c structure, where only bending and tilt modes are observed. The main peaks of compounds with regular MnO 6 octahedra, such as CaMnO 3 , highly Ca-doped LaMnO 3 , or the metallic phases of Ca-or Sr-doped LaMnO 3 , are bending and tilt MnO 6 octahedra modes which correlate to R-O(1) bonds and Mn-O-Mn angles, respectively. In low and optimally doped manganites, the intensity and width of the broad bands are related to the amplitude of the dynamic fluctuations produced by polaron hopping in the paramagnetic insulating regime. The activation energy, which is proportional to the polaron binding energy, is the measure of this amplitude. This study permits to detect and confirm the coexistence, in several compounds, of a paramagnetic matrix with lattice polaron together with regions without dynamic or static octahedron distortions, identical to the ferromagnetic metallic phase. We show that Raman spectroscopy is an excellent tool to obtain information on the local structure of the different microphases or macrophases present simultaneously in many manganites.

Perovskite La1−x Sr x Ga1−y Mn y O3 solid solution crystals: Raman spectroscopy characterization

“…B 3g ) and two bending modes (A g ? B 1g ) [12]. As confirmed in papers [12,14,20], the modes of B 2g and B 3g symmetry have very low intensity.…”

“…However, the RT investigation of La 1-x Ca x MnO 3 compounds reported in [12,16,17] shows significant changes in Raman spectra with increasing Ca content. The spectrum of the OR LaMnO 3 compound shows two intensive spectral features at 480 and 610 cm -1 and one weak band at 280 cm -1 , while the spectrum of the paramagnetic CaMnO 3 compound of the same OR Pbnm space group is much different.…”

“…5B 2g ? 5B 3g ) [11][12][13]. These modes can be classified into two symmetric and four antisymmetric stretching modes, four bending modes and six rotation and tilt modes of the octahedra.…”

Raman spectra in epitaxial thin films ofLa1−xCaxMnO3(Xiong1, Chen2, Wang3 et al. 2004Phys. Rev. B502232View full textAdd to dashboardCite

“…According to the previous reports, the Raman feature ω 2 could contain several peaks with different symmetry 36,43,46 , its temperature dependence is com-plex due to the overlap of these modes. The different temperature behaviors of these modes induce a shift of the broad ω 2 band.…”

Percolative transition on ferromagnetic insulator manganites: Uncorrelated to correlated polaron clusters

“…Recently, the nature of such local distortions, their dynamics and correlations have been addressed by several authors. [6][7][8][9][10][11] In spite of such an effort, several issues as the detailed structure of polarons, the temperature evolution of polaron clusters, or the effect of such evolution on the average macroscopic lattice structure still remain as open questions.Local distortions and their dynamics can be studied by using ␥-␥ perturbed angular correlation spectroscopy ͑PAC͒, a nuclear hyperfine method specially effective to sample atomic-scale environments. PAC efficiency is T independent, allowing us to explore a wide range of temperatures.…”